Hi, All.

Can anyone on the board tell me (from 1st hand experience, preferably) the specific symptoms that a bad knock sensor causes at both idle and running speeds? The truck is a '87 F150 I6 fuel-injected 300CID (4.9L) with a 4-speed manual and 137,000 miles.

I've tried a search here on the FTE Forum using both "knock" and "knock sensor" as the keyword, but either get 1000's of hits or no relevant ones.

With all that's been done to this engine over the last year, it should be the best running I6 in the world, but it still has what appears to be an engine mis-fire at both idle and running speed after it warms up. You can see it in the shake of the gearshift ****, and feel it in the cab, too.

I won't go into the specific details of everything I've done to this engine, as the list would be 'way too long. I'll just say there's no code showing up on the ECM, compression is within +-5 psi on all cylinders (after a valve job), NO VACUUM LEAKS, all new or tested sensors (except the knock sensor), IAC cleaned, EACH injector physically removed and cleaner manually forced through it, new O2 sensor, EGR valve blocked off, new ignition module, wiring, plugs, coil, rotor and cap, etc. I've even had the ECM out to ensure there's not a bad connection in the firewall plug connector somewhere. (Now there's a fun job -- trying to get the weather-warped rectangular gasket reseated correctly in the firewall around the ECM!!).

Everyone says this I6 300 engine is "bullet-proof". That may indeed be the case from a purely mechanical viewpoint, but the methods used for controlling and diagnosing engine fuel and ignition control problems sure leave a LOT to be desired.

Thanks in advance for any help!

 

I've found the built-in engine management & diagnosis systems to be superb...

You need to isolate the miss as a mechanical or ignition fault, and go from there. The knock sensor only tells the EEC to retard the timing slightly and momentarily, but you can easily identify a bad one by simply unplugging it. If the problem clears up instantly, the KS was overactive. If not, plug it back in & look elsewhere.

Are you using the Haynes manual? I find it the best single reference. Are you using this procedure to pull codes?